The present invention generally relates optical lithography and more particularly to the design layout and fabrication of transparent or semitransparent phase-shifting masks or reticles that can be used in the manufacture of semiconductor devices.
In lithography, an exposure energy, such as ultraviolet light, is passed through a mask (or reticle) and onto a target such as a silicon wafer. The reticle typically may contain opaque and transparent regions formed in a predetermined pattern. The exposure energy exposes the reticle pattern on a layer of resist formed on the target. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. This forms a resist mask. A mask typically may comprise a transparent plate such as fused silica having opaque (chrome) elements on the plate used to define a pattern. A radiation source illuminates the mask according to well-known methods. The radiation transmitted through the mask and exposure tool projection optics forms a diffraction limited latent image of the mask features on the photoresist. The resist mask can then be used in subsequent fabrication processes. In semiconductor manufacturing, such a resist mask can be used in deposition, etching, or ion implantation processes, to form integrated circuits having very small features.
As semiconductor manufacturing advances to ultra-large scale integration (ULSI), the devices on semiconductor wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges, of various features.
As the nominal minimum feature sizes continue to decrease, control of the variability of these feature sizes becomes more critical. For example, the sensitivity of given critical dimensions of patterned features to exposure tool and mask manufacturing imperfections as well as resist and thin films process variability is becoming more significant. In order to continue to develop manufacturable processes in light of the limited ability to reduce the variability of exposure tool and mask manufacturing parameters, it is desirable to reduce the sensitivity of critical dimensions of patterned features to these parameters.
As feature sizes decrease, semiconductor devices are typically less expensive to manufacture and have higher performance. In order to produce smaller feature sizes, an exposure tool having adequate resolution and depth of focus at least as deep as the thickness of the photoresist layer is desired. For exposure tools that use conventional or oblique illumination, better resolution can be achieved by lowering the wavelength of the exposing radiation or by increasing the numerical aperture of the exposure tool, but the smaller resolution gained by increasing the numerical aperture is typically at the expense of a decrease in the depth of focus for minimally resolved features. This constraint presents a difficult problem in reducing the patterning resolution for a given radiation wavelength.
A reduction projection exposure method that features mass-producibility and excellent resolution has been used widely for forming patterned features. According to this method, the resolution varies in proportion to the exposure wavelength and varies in inverse proportion to the numerical aperture (NA) of the projection optical system. The NA is a measure of a lens"" capability to collect diffracted light from a mask and project it onto the wafer. The resolution limit R (nm) in a photolithography technique using a reduction exposure method is described by the following equation:
R=K1xcex/(NA)
Where:
xcex is the wavelength (nm) of the exposure light;
NA is the numerical aperture of the lens; and
K1 is a constant dependent on a type of resist.
So far, increases in the resolution limit have been achieved by increasing the numerical aperture (high NA). This method, however, is approaching its limit due to, a decrease in the depth of focus, difficulty in the design of lenses, and complexity in the lens fabrication technology itself. In recent years, therefore, attention has been given to an approach for shortening the wavelength of the exposure light in order to form finer patterns to support an increase in the integration density of LSIs. For example, a 1-Gbit DRAM requires a 0.2-micrometer pattern while a 4-Gbit DRAM requires a 0.1-micrometer pattern. In order to realize these patterns, exposure light having shorter wavelengths must be used.
However, because of increased semiconductor device complexity that results in increased pattern complexity, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore, making diffraction a severe limiting factor for optical photolithography.
A conventional method of dealing with diffraction effects in optical photolithography is achieved by using a phase shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase shifting is a change in timing of a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material.
Typically, phase-shifting is achieved by passing light through areas of a transparent material of either differing thickness or through materials with different refractive indexes, or both, thereby changing the phase or the periodic pattern of the light wave. Phase shift masks reduce diffraction effects by combining both diffracted light and phase shifted diffracted light so that constructive and destructive interference takes place favorably. On the average, a minimum width of a pattern resolved by using a phase shifting mask is about half the width of a pattern resolved by using an ordinary mask.
There are several different types of phase shift structures. These types include: alternating aperture phase shift structures, subresolution phase shift structures, rim phase shift structures, and chromeless phase shift structures. xe2x80x9cAlternating Phase Shiftingxe2x80x9d is a spatial frequency reduction concept characterized by a pattern of features alternately covered by a phase shifting layer. xe2x80x9cSubresolution Phase Shiftingxe2x80x9d promotes edge intensity cut off by placing a subresolution feature adjacent to a primary image and covering it with a phase shifting layer. xe2x80x9cRim Phase Shiftingxe2x80x9d overhangs a phase shifter over a chrome mask pattern.
In general, these phase shift structures are constructed on reticles (or masks) having three distinct layers of material. An opaque layer is patterned to form light blocking areas that allow none of the exposure light to pass through. A transparent layer, typically the substrate, is patterned with light transmissive areas, which allow close to 100% of the exposure light to pass through. A phase shift layer is patterned with phase shift areas which allow close to 100% of the exposure light to pass through but phase shifted by 180xc2x0 (xcfx80). The transmissive and phase shifting areas are situated such that exposure light diffracted through each area is canceled out in a darkened area therebetween. This creates the pattern of dark and bright areas, which can be used to clearly delineate features. These features are typically defined by the opaque layer (i.e., opaque features) or by openings in the opaque layer (i.e., clear features).
For semiconductor manufacture, alternating aperture phase shift reticles may typically be used where there are a number of pairs of closely packed opaque features. However, in situations where a feature is too far away from an adjacent feature to provide phase shifting, subresolution phase shift structures typically may be employed. Subresolution phase shift structures typically may be used for isolated features such as contact holes and line openings, wherein the phase shift structures may include assist-slots or outrigger structures on the sides of a feature. Subresolution phase shift structures are below the resolution limit of the lithographic system and therefore do not print on the target. One shortcoming of subresolution phase shift structures is that they require a relatively large amount of real estate on the reticle.
Rim phase shifting reticles include phase shift structures that are formed at the rim of features defined by opaque areas of the reticle. One problem with rim phase shifter structures is that they are difficult to manufacture. In the case of rim phase shift structures, multiple lithographic steps must be used to uncover the opaque layer so that it can be etched away in the area of the rim phase shifter. This step is difficult as the resist used in the lithographic step covers not only the opaque layer but also trenches etched into the substrate.
In general, improvement of the integration density of semiconductor integrated circuits in recent years has been achieved mainly through a reduction in size of the various circuit patterns. These circuit patterns are presently formed mainly by optical lithography processes using a wafer stepper.
FIG. 1 shows the structure of such a prior art stepper. Mask 102 is illuminated by the light emitted from illumination system 102. An image of mask 108 is projected onto a photoresist film coated on wafer 120 which is the substrate to be exposed through projection system 110. As shown in FIG. 1, illumination system 102 includes a source 100, condenser lens 104, and aperture 106 for specifying the shape and size of the effective source. Projection system 110 includes a projection lens 112, pupil filter 114, and aperture 116 arranged in or near the pupil plane of focussing lens 118 to set the numerical aperture (NA) of the lens.
As discussed earlier, the minimum feature size R of patterns transferable by an optical system is approximately proportional to the wavelength xcex of the light used for exposure and inversely proportional to the numerical aperture (NA) of the projection optical system. Therefore, size R is expressed as R=k1xcex/NA, where k1 is an empirical constant and k1=0.61 is referred to as the Rayleigh limit.
In general, when the pattern dimensions approach the Rayleigh limit, the projected image is no longer a faithful reproduction of the mask pattern shape. This phenomenon is known as optical proximity effects and results in corner rounding, line-end shortening, and line width errors, among other things. To solve this problem, algorithms have been proposed that can be used to pre-distort the mask pattern so that the shape of a projected image takes on the desired shape.
Moreover, approaches have been described which improve the resolution limit of a given optical system, resulting effectively in a decreased value of k1. Adoption of a phase shifting mask, such as described above, is a typical example of this approach. A phase shifting mask is used to provide a phase difference between adjacent apertures of a conventional mask.
A chromeless phase shifting mask method is known as a phase shifting method suitable for the transfer of a fine isolated opaque line pattern, which is needed, for example, for the gate pattern of a logic LSI.
Off-axis illumination and pupil filtering are methods additionally known for improving images. According to the off-axis illumination method, the transmittance of aperture 106 is modified in the illumination system 102 of FIG. 1. One particular embodiment of this method changes the illumination intensity profile so that the transmittance at the margin becomes larger than that of the central portion, which is particularly effective to improve the resolution of a periodic pattern and the depth of focus. The pupil filtering method is a method of performing exposure through a filter (pupil filter) located at the pupil position of a projection lens to locally change the amplitude and phase of the transmitted light. For example, this approach makes it possible to greatly increase the depth of focus of an isolated pattern. Furthermore, it is well known that the resolution of a periodic pattern can further be improved by combining the off-axis illumination method and the pupil filtering method.
Nonetheless, an inherent problem with a conventional transmission mask, such as the ones described above, is that the substrate undergoes a decrease in transmissivity as the wavelength of light emitted from an exposure light source is decreased to obtain finer patterns. For example, a quartz material substrate becomes more opaque as the wavelength of the light source decreases, particularly when the wavelength is less than 200 nm. This decrease in transmissivity affects the ability to obtain finer resolution patterns. For this reason, a material for a transmission phase shifting mask which can obtain a high transmissivity with respect to light having a short wavelength is needed. It is, however, difficult to find or manufacture such a material having a high transmissivity with respect to short wavelength exposure light.
An example of photomask pattern is shown in FIG. 2. Passage of radiation around these features causes diffraction of the radiation into discrete dark and bright areas. The bright areas are known as the diffraction orders and the collective pattern they form is mathematically describable by taking the Fourier transform of the collective opaque and transparent region. The pattern that is observed in its simplest personification has an intense diffraction order, called the 0th order, surrounded in a symmetrical fashion by less intense diffraction orders. These less intense orders are called the plus, minus first (xc2x11st) order; plus, minus second (xc2x12nd) order on into an infinity of orders. For the same feature width, different diffraction patterns are formed for dense and isolated features. FIG. 3(A) shows the magnitudes of relative electric fields and respective pupil positions of diffraction orders for a dense feature, while FIG. 3(B) shows the magnitudes of diffraction orders for an isolated one. The center peak observed at each plot is the 0th order.
The 0th order contains no information about the pattern from which it arose. The information about the pattern is contained in the non-zero orders. However, the 0th order is spatially coherent with the higher orders so that when the beams are redirected to a point of focus they interfere, and in doing so construct an image of the original pattern of opaque and transparent objects. If all the diffraction orders are collected a perfect representation of the starting object is imaged. However, in high-resolution lithography of small pitch features, where pitch is the sum of the width of the opaque and transparent objects, only the 0th and the xc2x11st orders are collected by the projection lens to form the image. This is because higher orders are diffracted at higher angles that fall outside of the lens pupil as defined by numerical aperture (NA).
As depicted in FIG. 4(A), the 0th order 402 and the xc2x11st orders 404 lie within the lens pupil 406. As further depicted in FIG. 4(A), the xc2x12nd orders 408, lie outside the lens pupil 406. Further, as seen in FIG. 4(B), a corresponding aerial image is formed during exposure. The photo resist pattern is then delineated from this aerial image.
It has long been known that it is only necessary to collect two diffraction orders, such as either with the 0th order and at least one of higher diffraction orders, or simply with two higher orders without the 0th order, to form the image.
As depicted in FIG. 5(A), light transmitted through a focussing lens 502 is represented by that which is normal 504 to the object (not shown), and that which transmits through the edges 506, 508 of the focussing lens 502. Although light is continuously transmitted throughout the entire surface of lens 502, the three light paths 504-508 are represented to illustrate phase matching of different light paths. At point 510, the three light paths 504-508 focus and are in phase together. When three light paths 504, 512, and 514 focus together at point 516, however, they are not in phase. The phase error from a change in path-lengths of 512 and 514 from respective path-lengths 506 and 508 results in a finite depth of focus, DoF, of the system.
One may improve the tolerance to variations in relative phase error caused by aberrations like defocus as depicted in FIG. 5(A). FIG. 5(B) represents how by eliminating the light path that is normal to the object, variations to the phase error may be reduced. Again, although light is continuously transmitted throughout the surface of lens 502, the two light paths 506 and 508 are represented to illustrate phase matching of different light paths. At point 510, the two light paths 506 and 508 focus and are in phase together. When the two light paths 512 and 514 focus together at point 516, they are in phase. Without the light path 504 as seen in FIG. 5(A), the phase error from the increased path-lengths of 512 and 514 over respective path-lengths 506 and 508 is eliminated and results in an infinite depth of focus, DoF, of the system. Eliminating the light path normal to the object may be accomplished by placing an obscuration in the center of the radiation source thus eliminating light normal to the object and allowing only oblique illumination, as depicted for example in FIGS. 6(A) and 6(B).
FIG. 6(A) depicts a lithographic xe2x80x9con-axisxe2x80x9d projection system wherein the reticle 602 permits transmission of the light path normal to the object. In the figure, light passes through the reticle 602, comprising a glass substrate 604 and chrome patterns 606, through the lens aperture 608, into lens 610, and is focused into area 612. FIG. 6(B) depicts exemplary lithographic xe2x80x9coff-axisxe2x80x9d projection systems wherein the an annular reticle 614, or quadrupole reticle 616, prohibits transmission of the light path normal to the object. In the figure, light passes through the glass substrate 604, past the chrome patterns 606, through the lens aperture 608, into lens 610, and is focused into area 618. Comparing FIGS. 6(A) and 6(B), it is noted that the DoF of FIG. 6(A) is smaller than that of FIG. 6(B).
Lowering the 0th order""s magnitude to be the same or less than that of the 1st order improves the imaging tolerance of this two beam imaging system. One method for tuning the magnitude of the diffraction orders is to use weak phase shift masks. Strong phase shift masks and weak phase shift masks differ in operation and effect.
Strong phase shift masks eliminate the zero-diffraction order and double the resolution through a technique of frequency doubling. To understand how strong shifters work, it is useful to think of the critical pitch as having alternating clear areas adjacent to the main opaque feature. Because of the alternating phase regions, the pitch between same phase regions is doubled. This doubling, halves the position the diffraction orders would otherwise pass through the projection lens relative to the critical pitch; thus making it possible to image features with half the pitch allowed by conventional imaging. When the two opposing phase regions add through destructive interference, to build the final image, the magnitude of their respective zero order light is equal in magnitude but of opposite phase, thus canceling each other. Imaging is done only with the frequency doubled higher orders. On the other hand, weak phase shift masks dampen the zero-order light and enhance the higher orders. Weak phase shift masks form their phase shift between adjacent features by creating electric fields of unequal magnitude and of opposite phase, with the field immediately adjacent to a critical feature having the lesser of the magnitudes. The net electric field reduces the magnitude of the zero order while maintaining the appropriate phase.
Weak phase shift masks permit an amount of exposure radiation to pass through objects in a fashion that creates a difference in phase between coherently linked points while having an imbalance in the electric field between the shifted regions. FIG. 7(A) depicts a substrate 702 and a mask 704 that does not permit phase shifting. FIG. 7(C) is a graph illustrating how the 0th order""s magnitude is larger than that of the xc2x11st orders"" magnitude from a non-phase shifting mask as depicted in FIG. 7(A). FIG. 7(B) depicts a substrate 702 and a mask 706 that permits phase shifting. FIG. 7(D) is a graph illustrating how the 0th order""s magnitude is decreased to be comparable to that of the xc2x11st orders"" magnitude from a phase shifting mask as depicted in FIG. 7(B).
Several types of phase-shifting masks are known in the art as the rim, attenuated or embedded (or incorrectly halftone), and unattenuated or chromeless (or transparent) shifter-shutter phase-shifting masks.
FIG. 8(A) is a cross-sectional view of a rim phase-shifting mask 802, comprising a light transmitting portion 804, and a light inhibiting portion 806. FIG. 8(B) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 8(C) is a diagram representing the magnitude of the 0th diffraction order 810, and xc2x11st orders 812, 814, coinciding with the respective pupil positions as depicted in FIG. 8(A).
FIG. 9(A) is a cross-sectional view of an attenuated or embedded phase-shifting mask 902 having an attenuation of 5%, comprising a light attenuating portion 904. FIG. 9(B) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 9(C) is a diagram representing the magnitude of the 0th diffraction order, and xc2x11st diffraction orders coinciding with the respective pupil positions as depicted in FIG. 9(A). FIG. 9(D) is a cross-sectional view of an attenuated or embedded phase-shifting mask 912 having an attenuation of 10%, comprising a light attenuating portion 914. FIG. 9(E) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 9(F) is a diagram representing the magnitude of the 0th diffraction order, and xc2x11st diffraction orders, coinciding with the respective pupil positions as depicted in FIG. 9(D).
FIG. 10(A) is a cross-sectional view of an unattenuated or chromeless (or transparent) shifter-shutter phase-shifting mask 1002, comprising a light-shifting portion 1004. FIG. 10(B) is a graph representing the amplitude of the E-field at the mask, whereas FIG. 10(C) is a diagram representing the magnitude of the 0th diffraction order 1006, and xc2x11st diffraction orders 1080, 1010, coinciding with the respective pupil positions as depicted in FIG. 10(A).
Typically, the phase-shifting masks of FIG. 8 through FIG. 10 form their phase-shift differently but relative to their non-phase-shifted counterpart, they all yield a 0th diffraction order of smaller amplitude and a first diffraction order of larger amplitude of its electric field. Which ratio of 1st to 0th diffraction order magnitude is optimal depends on the pitch of the feature being imaged along with the shape of the illuminator and the desired printing size in the developed photoresist. These tuned diffraction patterns are then used with off-axis illumination to image smaller pitches with better tolerance to imaging process variation.
The concept of manipulating of the amplitude ratio of 0th-1st diffraction orders has conventionally been restricted to using certain weak phase-shifting techniques with biasing features and sub-resolution assist features.
FIG. 11(A) depicts a conventional biasing technique used to resolve a desired feature. As seen in FIG. 11(A), biasing bars 1102 and 1104 are situated adjacent the mask of the primary feature 1106. FIG. 11(B) depicts a half-tone biasing technique known to the Applicants of the instant application and described in U.S. patent application Ser. No. 09/055,355 now U.S. Pat. No. 6,114,071, used to resolve a desired feature. As seen in FIG. 11(B), half-tone biasing bars 1108 and 1110 are situated adjacent the mask of the desired feature 1112. FIG. 12 depicts a conventional photoresist mask 1202. The photoresist mask 1202 comprises a plurality of scatter bars 1204, serifs 1206, and chrome shields 1208.
For conventional attenuated shifters, transparency of the shifter materials typically may be adjusted, and used along with biasing and sub-resolution assist features. Transparency of the shifters typically ranges from 3% to 10%, wherein higher transmissions such as from 10% to 100% are reported to be optimal for pitches where the space between the features is larger than the phase-shifted line. FIG. 13 shows the dependence of image contrast, as defined by the normalized-image-log-slope (NILS), with respect to varying transmittance of its phase-shifted material for a 175 nm line on a 525 nm pitch (FIG. 13A) and a 1050 nm pitch (FIG. 13B). Each curve in the figure represents a different focus setting. The curve with the largest NILS is the most focussed, and has a value of zero, and then with each change in focus the NILS of each respective curve decreases. FIG. 13A shows that the best transmission for the 175 nm line with the 525 nm pitch structure is 0.35 to 0.45. FIG. 13B shows that the best transmission for the 175 nm line with the 1050 nm pitch structure is 0.25 to 0.35.
An example of a 100% transparent attenuated phase-shifting technology is the previously mentioned, chromeless shifter-shutter, such as depicted in FIG. 10. Using a chromeless shifter-shutter, phase-edges of a pattern typically may be placed within an area that is 0.2 to 0.3 times the exposing wavelength xcex divided by the numerical aperture NA of the projection lens. For lines larger or smaller than this, the destructive interference is insufficient to prevent exposure in an area not be exposed. Printing features larger than this is accomplished in one of two ways. The first places an opaque layer in the region that is to stay dark with the feature edges being opaque or rim-shifted (FIG. 14). The second, as depicted in FIG. 15, creates a dark grating 1502 by placing a series of features 1504 whose size meets the criteria for printing an opaque line 1506 using chromeless technology.
Conventionally, chromeless phase shifting masks have not worked with off-axis exposure as the shifter (feature) sizes and shutter (space) sizes approach one another. FIGS. 16(A) through 16(C) depict a conventional chromeless phase shifting mask. In FIG. 16(A), 1602 is a cross-sectional view of a portion of a conventional chromeless phase shifting mask, comprising shifters 1604, and shutters 1606, wherein the shifter length is substantially equal to the shutter length. FIG. 16(B) is a graph representing the amplitude of the E-field at the mask 1602. FIG. 16(C) is a diagram representing the magnitudes of the xc2x11st diffraction orders 1608 and 1610 for the mask of FIG. 16(A). As seen in FIG. 16(C) there is no 0th diffraction order. The functional limit of the relative sizes of the shifter and shutters of conventional chromeless phase shifting masks results from the integrated electric fields of the two opposing phase-shifted regions being equal. This balanced condition cancels the 0th diffraction order making it impossible to get the prerequisite 0th diffraction order needed for using off-axis illumination.
To summarize, each of the above-described, conventional, weak phase-shifting techniques solved certain imaging problems. However, each technique has accompanying drawbacks. For example, the rim, attenuated or embedded, and unattenuated or chromeless (or transparent) shifter-shutter phase-shifting masks provided large ratios in the 0th-xc2x11st diffraction orders. Prior art attempts to manipulate these ratios included using biasing techniques coupled with an attenuated phase shifting mask. However, these prior art attempts included complex manufacturing steps and yielded inefficient masks as a result of the attenuation. Furthermore, unattenuated shifter-shutter phase-shifting masks additionally failed to yield accurate images with off-axis illumination as the shifter and shutter sizes approached one another.
It is an object of this invention to provide a simple system and method for fabricating an efficient phase shifting mask that is able to manipulate the ratios in the 0th-xc2x11st diffraction orders.
It is another object of this invention to provide a system and method for fabricating a non-attenuated phase shifting mask that is able to manipulate the ratios in the 0th-xc2x11st diffraction orders.
It is yet another object of this invention to provide a system and method for fabricating a chromeless (or transparent) shifter-shutter phase-shifting mask that is usable with off-axis illumination when the shifter and shutter sizes approach one another.
It is still another object of this invention to provide a system and method for halftoning primary features to achieve the correct ratio of 0th to higher diffraction order light for optimal imaging.
It is still yet another object of this invention to provide a system and method for halftoning assist features to achieve the correct ratio of 0th to higher diffraction order light for optimal imaging.
The present invention provides an alternate method for effectivey manipulating the amplitude ratio of the 0th-1st diffraction order by using halftoning of opaque and phase-shifted transparent/semi-transparent features within the primary feature and as sub-resolution assist features. The relative magnitudes of the 0th and higher diffraction orders are formed as the exposing wavelength passes through the plurality of zero and 180xc2x0 phase-shifted regions. Subsequently some of the diffraction orders are collected and projected to form the image of the object.
Methods in accordance with the present invention further make use of halftoning structures to manipulate the relative magnitudes of diffraction orders to ultimately construct the desired projected-image. At the resolution limit of the mask maker, this is especially useful for converting strong shifted, no 0th diffraction order, equal line and space chromeless phase edges to weak phase shifters that have some 0th order. Halftoning creates an imbalance in the electric field between the shifted regions and therefore results in the introduction of the 0th diffraction order. As such, with halftoning, these previously strong shifted features convert to weak phase-shifters and are compatible with the other shifter-shutter chromeless features typically found amongst the plurality of objects used in making a conventional semiconductor circuit.
Decreasing the size of the primary feature for the very dense features, as in the conventional mask fabrication technique, can achieve a limited extent of modifying diffraction order. Because of the interference effects, it is not possible to ensure that a mask width less than the sub-resolution assist feature can be reliably made using conventional mask fabrication methods. However, in accordance with the present invention, by biasing the primary features, the feature width can be reduced to less than the sub-resolution assist features.
Further, chromeless phase-shifting mask is known to be a powerful imaging method when combined with using off-axis illumination, but it has serious optical proximity effect. This invention provides an effective optical proximity solution.
In general, in one aspect, the invention features a method of transferring an image including 0th diffraction order and xc2x11st diffraction orders, onto a material, wherein the method comprises the steps of fabricating a phase shifting mask comprising at least one unattentuated, halftoned, phase-shift feature, and off-axis illuminating the mask such that light passes through the mask onto the material.
In another aspect, the invention features a method of transferring an image including 0th diffraction order and xc2x11st diffraction orders, onto a material, wherein the method comprises the steps of fabricating a phase shifting mask comprising at least one feature, wherein at least one feature includes halftoned phase-shifted transparent features; and off-axis illuminating the mask such that light passes through the mask onto the material. Preferably, one feature further includes semi/transparent features. Still preferably, the at least one feature further includes opaque features.
In yet another aspect, the invention features a phase shifting mask comprising at least two unattentuated, halftoned, phase-shift features having a width w, wherein the features are separated by a width w, such that the mask provides an image including 0th diffraction order and xc2x11st diffraction orders, when illuminated.
In still yet another aspect, the invention features a phase shifting mask comprising at least two halftoned phase-shifted transparent features having a width w, wherein the features are separated by a width w, such that the mask provides an image including 0th diffraction order and xc2x11st diffraction orders, when illuminated. Preferably, the at least two features further includes semi/transparent features. Still preferably, the at least two features further includes opaque features. Still yet preferably, a focus-exposure process window for maintaining a predetermined resist line-width sizing of the mask is common to an attentuated, phase-shift mask of a similar pitch.
As described in further detail below, the present invention provides significant advantages over the prior art. Most importantly, the unattenuated phase-shift photomask of the present invention allows for the printing of high resolution features, while manipulating the 0th diffraction order and xc2x11st diffraction orders.
In addition, because the unattenuated phase-shift mask of the present invention provides a focus-exposure process window for maintaining an increased line-width sizing over that of the prior art.
Additional advantages of the present invention will become apparent to those skilled in the art from the following detailed description of exemplary embodiments of the present invention. The invention itself, together with further objects and advantages, can be better understood by reference to the following detailed description and the accompanying drawings.